1. Field of the Invention
This invention describes a transporter for an ultra-fine alignment tool which measures changes in position by referencing an optical cavity to a frequency-stabilized light source. This invention particularly relates to a mounting system equipped with a dual-locking clamp which maintains the cavity alignment when the alignment tool is relocated to a different position and minimizes the need for realignment after each change.
2. Description of Related Art
A fine alignment system incorporating a frequency-modulation spectroscopic positioning system and a dual-frequency optical heterodyne detection technique has been described by K. Jain, one of the inventors of the present application, in U.S. Pat. No. 4,991,962, issued Feb. 12, 1991. That patent uses diffraction of targets and collection of the diffracted light patterns to achieve ultra-high precision placement of a mask with respect to a substrate for microlithography. That patent shows use of a Fabry-Perot resonator as a real-time positioning system.
In the majority of step-and-repeat microlithographic imaging systems, the pattern on the mask is imaged onto the wafer for many different relative positions of the mask and wafer. This enables the exposure of the large number of chips or groups of chips (.about.10-100) on the wafer using a mask size equal to the size of one imaged segment. After proper alignment and exposure of each segment, the mask and wafer are displaced relative to one another by the extent of one segment. The mask and wafer are aligned again and another exposure is then made so that a duplicate image is formed on the wafer at a location adjacent to the previous exposure. This process is repeated so that a grid of identical images is formed on the wafer.
Any change to the relative position of the mask and target substrate along either the x-axis or the z-axis (the exposure beam is along the y-axis) affects the cavity mirror spacing. The mirrors will either no longer form an optical cavity, or the new optical cavity will have a significantly different free spectral range, thereby disabling the alignment system. For movement along the axis in the plane of the mirror, this problem can be resolved by making one of the mirrors wide enough so as to maintain an optical cavity in each of the relative mask-wafer positions. This is a common technique used in laser interferometry. However, when either the mask or the wafer is displaced along a direction orthogonal to the plane of the Fabry-Perot mirror, the free spectral range of the Fabry-Perot cavity will change significantly, and the fine alignment system will cease to function properly. For the exposure of each segment, the optical cavity needs to be maintained at a fixed distance and orientation.